Everything homeowners need to know about building an accessory dwelling unit in Ontario, California - size limits, setbacks, permits, and how the Inland Empire's largest logistics hub handles by-right ADU approvals under state law.
| ADU type | Max size | Max height | Notes |
|---|---|---|---|
| Detached, single-family lot | 850 sq ft (studio/1-bedroom) or 1,000 sq ft (2+ bedrooms) | 16 ft generally; up to 18-20 ft within a half-mile of a major transit stop | No application of the size or lot-coverage formula may push a detached ADU below 800 sq ft |
| Attached, single-family lot | Same 850/1,000 sq ft caps, further limited to 50% of the primary dwelling's floor area | Up to 25 ft or the underlying zone's height limit, whichever is lower; capped at two stories | |
| Multifamily ADU (lots with 2+ attached units) | Up to 2 detached ADUs, plus conversion of non-habitable space for up to 25% of existing units | 18 ft for a detached ADU on a lot with a multi-story multifamily building | An attached ADU is not allowed on multifamily lots |
| Junior ADU (JADU) | Up to 500 sq ft | Limited to the height of the existing structure | Must be within an existing or proposed single-family home or attached garage; not permitted on lots with multiple detached single-family dwellings |
ADUs and JADUs are permitted on lots with an existing or proposed single-family or multifamily dwelling, and may also be permitted in non-residential zones if the lot already contains a legally permitted dwelling unit. Confirm your parcel's zoning with the Planning Department before designing.
| Setbacks | Front yard follows the underlying zoning district; 4 ft for rear, interior side, and street side yards. No setback is required for a conversion built in the same location and dimensions as an existing structure. A 6 ft separation is required between detached structures. |
| Parking | 1 off-street space required per ADU, which may be provided as tandem parking or within a setback area. Ontario's code lists exceptions (Section 5.03.010(F)(6)(b)) that track the state law exemptions, such as proximity to transit. Replacement parking is not required when an ADU or JADU takes over required parking for the primary home. |
| Units per lot | 1 ADU (attached or detached) plus 1 JADU on a single-family lot; multifamily lots can add 2 detached ADUs and/or conversions of non-habitable space up to 25% of existing units |
| Owner-occupancy | Not required for a standard ADU. A JADU requires the owner to occupy either the primary residence or the JADU. |
| Building code | An ADU must have its own kitchen and bathroom; a JADU needs an efficiency kitchen and either separate or shared sanitation facilities. Title 24 energy compliance documentation is required at submittal. |
| Rental restriction | Units may not be used as short-term rentals and may only be rented for terms of 30 days or longer |
| Impact fees | Required for an ADU or JADU that is 750 sq ft or larger; waived below that threshold under statewide SB 13 |
Worth knowing: as of the most recent update to the state's public ADU ordinance-review tracker, Ontario does not appear on the California Department of Housing and Community Development's list of cities that have received an ADU ordinance findings or non-compliance letter, unlike some neighboring Inland Empire jurisdictions. That's a reasonable signal the city's standards track state law, though it's still worth confirming specifics with Planning before you design.
Call the Planning Department at (909) 395-2036 or email PlanningCounter@ontarioca.gov to confirm your parcel's zoning and which size tier applies.
You'll need a complete site plan, floor plans and elevations, and Title 24 energy compliance documentation.
Building permit applications and all submittal requirements go in electronically through the city's Citizen Portal Access website.
The city reviews for zoning and building code compliance. As with all California cities, state law caps ministerial ADU review at 60 days for a complete application.
Impact fees apply once the ADU or JADU reaches 750 sq ft. The Building Department, (909) 395-2023 or BuildingCounter@ontarioca.gov, can walk you through what's owed for your specific project.
Begin construction once your permit is issued and schedule the standard framing, plumbing, electrical, and final inspections before occupancy.
Fees and timelines are approximate - confirm current figures with the Planning or Building Department before you apply.
| Garage conversion | Roughly $60,000-$150,000, the least expensive path since the shell already exists |
| New detached ADU | Roughly $130,000-$280,000 for a code-compliant, freestanding unit, depending heavily on size and finishes |
| Parking space (if required) | Budget for a paved off-street space unless your lot qualifies for a state-law exemption, such as proximity to transit |
| Impact fees | Waived for ADUs and JADUs under 750 sq ft; charged proportionately above that threshold under the Mitigation Fee Act |
These are general ranges from local contractor estimates and regional reporting, not quotes - actual cost depends heavily on lot conditions, finishes, and whether a parking space needs to be added. Inland Empire construction costs generally run below coastal Southern California.
